An electromagnetic (EM) radiation beam traversing a uniform medium or vacuum tends to spatially spread which steadily weakens the beam as it progresses through the medium. Means to confine the beam over great distances include an optical fiber made of glass. However, when the power levels of the EM radiation are sufficiently large, the fiber is destroyed. Moreover, glass fibers cannot transmit x-rays effectively. The present invention provides a new technique and applications for high intensity optical and short wavelength EM transmissions through a gaseous medium without EM beam spatial spreading, and in particular, creating a gaseous optical guiding technique over distances well in excess of a Rayleigh length, which is defined as a characteristic distance that a focused light beam of a certain diameter and wavelength can travel before transverse spreading occurs, by using a prepared plasma refractive index channel.
There is a strong interest in channeling high-intensity laser pulses through plasmas over distances exceeding a Rayleigh length. An early proposed application for channeling was nuclear fusion using long pulse or CW lasers. Recent suggested applications, using intense short pulses, include pumping of x-ray lasers and laser-plasma based particle accelerators. Experiments involving nonlinear processes, such as high harmonic generation, also benefit from an increased interaction length. The present invention has applications in all these areas as presented below.
One approach to channeling of intense pulses, suggested by recent advances in intense pulse laser technology, relies on self-induced modifications of the plasma refractive index. The on-axis enhancement of refractive index by relativistic electron dynamics or by ponderomotive force-driven charge displacement requires extremely high intensities (above 10.sup.19 W/cm.sup.2). Although this approach has not been demonstrated, recent calculations show that such an intense short pulse is subject to erosion: the leading edge diffracts due to a balance between forward charge displacement and relativistic electron response and the trailing edge is scattered by Raman instabilities. Even more fundamentally, it is not clear that such focused intensities can even be achieved in a medium: beams approaching the focus can refract due to plasma formation or undergo filamentation, at much lower thresholds than the onset of charge displacement or relativistic effects. The use of prepared index structures however, as outlined in the present invention, provides a means of stable channeling.
The present invention provides a method and means for true channeled guiding of intense optical pulses over distances in excess of a Rayleigh length. As shown later herein, the means also can provide for channeled propagation of x-rays. Prior art that pertains to two application areas of this invention include x-ray lasers and electron accelerators. The main difference of the present invention with the prior art is that the instant invention employs guiding electromagnetic beams (both optical and x-ray) as its main feature, while waveguiding is not obtained or desired by the prior art. Previous patents include Schafer's U.S. Pat. No. 4,630,274 which gives a method and apparatus for generating a hot plasma which emits EM radiation in a wavelength range below about 100 nm in a predetermined volume, in which the laser pulse is of short duration with sufficient energy and power density focused into the volume containing the target material. However, this patent does not teach or suggest applicants' method of creating a light pipe and its' attendant applications of using high intensity optical and x-ray waveguiding capabilities of long heated plasmas once they expand into neutral or weakly ionized gas. This is the central feature of the instant invention's technique, upon which the applications depend. In addition, Schafer's '274 patent recommends creating the plasma with a sub-picosecond pulse, preferably below 0.2 ps. The instant inventions's waveguide creation requires pulses in excess of 1-10 ps or sequences of shorter pulses with sequence duration in excess of 1-10 ps, but in general, no longer than about 1-10 ns. This provides: (1) an axially uniform breakdown or spark which cannot without difficulty be produced with a 0.2 ps pulse, since axicon surface quality requirements are severe and (2) sufficient heating so that the plasma generates a shock and expands to produce a waveguide. In addition, Schafer's '274 patent recommends use of a short wavelength laser (typically an excimer UV laser) to make the long plasma. The instant invention has no such restriction on the laser wavelength. A wide range of wavelengths is appropriate using the instant invention's technique. Moreover, Schafer's '274 patent refers to traveling wave excitation, but this does not refer to the instant invention's process of guiding waves by the plasma. The traveling wave excitation of Schafer's '274 patent refers to the convergence of the axicon-generated conical wave onto the optical axis, which has no bearing upon a guided beam. The instant invention's use of the term traveling wave excitation refers to the excitation provided by the guided pulse as it propagates in the plasma waveguide.
Another previous patent is the Rosen et al. U.S. Pat. No. 5,016,250 of an x-ray laser device that uses a two pulse laser technique provided by optical laser means of relatively low energy and small physical size. The differences with the instant invention are as follows: The Rosen et al. '250 U.S. patent's first pulse prepares a "narrow and linear plasma of uniform composition" along a flat thin foil, which is then heated by a second pulse which is swept across it at the speed of light by means of diffraction from a diffraction grating. Moreover, the Rosen et al. '250 U.S. patent mentions nothing about optical guiding, which is central to the instant invention. The instant invention's first pulse (or pulse sequence) does not heat a flat foil, but heats a gaseous plasma or solid blade edge unlike in the Rosen et al. '250 U.S. patent. The instant invention's first plasma is radially nonuniform for it to effectively act as a waveguide. In the Rosen et al. '250 U.S. patent, the second pulse is focused by a cylindrical lens, along one side and exterior to the first plasma. The instant invention's second pulse (or pulse sequence) is guided along the first plasma (which is a waveguide) and is internal to it, allowing huge enhancements in heating efficiency and repetition rate (see section C (a)). The Rosen et al. '250 U.S. patent specifies energies of 15 joules and 54 joules respectively in the first and second pulses, while our invention requires only in excess of 10 millijoules in either pulse.
Another previous patent is Suckewer's U.S. Pat. No. 4,704,718, which makes no mention of optical guiding, the crucial feature of the instant invention. In the Suckewer patent, two optical laser pulses are used to make an x-ray laser (first pulse: duration 10-100 ns and 1.5 kJ energy; second pulse: 1-2 ps duration and intensity 10.sup.15 W/cm.sup.2). The Suckewer patent recommends focusing both pulses from the side. The plasma is confined by an external magnetic field. The instant invention, however recommends use of a Bessel beam (although other means could be used) to prepare a plasma, and a second pulse to be optically guided by this plasma, where the second pulse is injected along the refractive index channel. No magnetic field is required for the instant invention, and as stated earlier, the channel creation pulse can operate with as little as 10 mJ, and should be shorter than 1-10 ns, as opposed to the 10-100 ns duration of the first pulse of the Suckewer invention.